Optical sensors find ever increasing application in various devices like smartphones, tablets and other mobile devices. At the same time there is a constant demand to reduce size of these devices and optical sensor design needs to keep the pace.
Optical lenses are often indispensable elements to optical sensor design and are a major parameter defining the overall height of the sensor. There have been many attempts to reduce the height of optical lenses, such as using Fresnel lenses for example. However, the dimension of optical lenses cannot be arbitrarily reduced without losing optical quality. For example, because of the diffraction limit lenses cannot be made smaller without severely deteriorating resolution. Furthermore, size reduction may introduce unwanted optical aberrations such as chromatic and spherical aberration. In the past optical aberrations have been accounted for by hybrid structures comprising one or more refractive and diffractive lens elements. However, the resulting build height still leaves room for improvement.
Alternatively or in combination with the optical lenses, diffractive designs have been used in order to do reduce height. FIG. 5B shows the position of a focal spot on the focal plane for a purely diffractive design. The position relative to the focal plane (shown as a dashed line) is depicted on the X axis. A power flow in arbitrary units is depicted in the Y axis. Typically, the diffractive lens design is optimized for a given wavelength λ. Correspondingly in focus the highest power flow can be seen on the position of the focal plane. However at slightly different wavelengths, i.e. 0.85λ and 1.15λ the highest power flow is far away from the focal plane. This is a clear indication of chromatic aberration.